The invention relates to an improved process for separating substantially pure ammonia and substantially pure carbon dioxide from mixtures containing ammonia, carbon dioxide and, possibly, water.
In some chemical processes mixtures containing ammonia and carbon dioxide, and sometimes also containing water, are obtained as by-products. For instance, in the synthesis of melamine from urea, a gas mixture is obtained which, in addition to melamine, also contains ammonia and carbon dioxide in amounts of at least 1.7 tons per ton of melamine. Such mixtures are also obtained in the preparation of urea from ammonia and carbon dioxide, resulting from the decomposition of by-product ammonia carbamate, and its separation from the urea product. In order to effectively utilize this ammonia and carbon dioxide after separating it from the melamine or urea, for example as recycle to a urea synthesis process, it is in most cases necessary to raise the gases to a higher pressure. Compression of such mixture requires special measures to prevent the condensation of ammonia and carbon dioxide and the deposition of solid ammonium carbamate thereby formed.
For this reason, such gas mixtures are usually absorbed in water or in an aqueous solution, which results in the formation of an ammonium carbamate solution which can be pumped to the urea synthesis reactor, sometimes being concentrated by desorption and repeated absorption at a higher pressure. A disadvantage of this procedure is that the water, recycled to the urea reactor together with the ammonia and carbon dioxide, has an unfavorable effect on the urea synthesis reaction.
It has been proposed to separately remove the ammonia and carbon dioxide from the by-product mixtures, and to separately recycle them in order to avoid the formation and deposition of ammonium carbamate. However, the binary system of ammonia and carbon dioxide forms a maximum boiling azeotrope at a molar ammonia-to-carbon dioxide ratio of about 2:1, and therefore cannot be separated by simple distillation. This phenomenon also occurs in the ternary system of ammonia, carbon dioxide and water, and the term azeotrope as used herein should be understood to include this phenomenon in the ternary system as well. Also as used herein, with respect to such binary or ternary mixture, the term "rich" with respect to ammonia shall be understood to mean that when heat is applied to a mixture `rich` in ammonia, substantially pure gaseous ammonia escapes, until the remaining mixture has a composition on the boundary line (to be defined hereinafter). On the other hand the term `lean` with respect to ammonia means that the mixture is not `rich` in ammonia.
Conversely the term `rich` with respect to carbon dioxide means that when heat is applied to a mixture `rich` in carbon dioxide, substantially pure carbon dioxide escapes. The term `lean` with respect to carbon dioxide means, that the mixture is not `rich` in carbon dioxide.
FIG. 1 shows the ammonia-carbon dioxide-water ternary system at constant pressure in a triangular diagram. The system is divided into two areas by line III which is termed herein the "boundary line", and which represents the azeotropic composition at constant pressure at varying water concentrations. This boundary line cannot be transgressed by means of normal distilling or rectifying techniques.
Thus when a liquid mixture rich with respect to ammonia, that is, falling within the area I on FIG. 1, is rectified, gaseous ammonia escapes until the liquid composition reaches the boundary line. When a mixture rich with respect to carbon dioxide, falling within area II on FIG. 1 is rectified, gaseous carbon dioxide escapes until the liquid composition reaches the boundary line. Once the liquid mixture composition is at the boundary line, further rectification or distillation at constant pressure results in a gaseous mixture of all components, but the composition of the remaining liquid mixture does not leave the boundary line. See also, P. J. C. Kaasenbrood, Chemical Reaction Engineering, Proceedings of the Fourth European Symposium, Sept. 9-11, 1968, published by Pergamon Press (1971), pages 317-328.
Various methods have been proposed to get around this azeotrope, all of which entail the separation of the ammonia-carbon dioxide mixtures into their constituents. Separate recovery of ammonia is most important, it being the most valuable component.
Some of these processes are based on selective absorption of either the ammonia or the carbon dioxide in a liquid. The Netherlands patent application No. 143,063, for example, describes a process in which ammonia is absorbed in an aqueous solution of an ammonium salt of a strong acid, such as ammonium nitrate, at an elevated pressure. Selective absorption of carbon dioxide by washing a gas mixture with an aqueous alkanolamine solution, such as monoethanolamine is disclosed in German Pat. No. 669,314. However, all of these processes have the drawback that the absorbed component must thereafter be removed from the absorbent and purified.
It has further been proposed to separate ammonia and carbon dioxide from mixtures of ammonia, carbon dioxide and water by distilling off most of the ammonia in a first step followed by distilling off the carbon dioxide in a second step carried out at a higher system pressure. The term "system pressure" as used herein means the sum of the partial pressures of ammonia, carbon dioxide and water. Processes of this kind are described in U.S. Pat. Nos. 3,112,177 and 4,060,591 and in British Pat. Nos. 916,945 and 1,129,939.
U.S. Pat. No. 3,112,177 describes a process in which in a first step carried out at a system pressure of between 1 and 5 atmospheres absolute, carbon dioxide gas is separated from a mixture of ammonia, carbon dioxide and water, which mixture is lean with respect to ammonia. The remaining liquid is then stripped with, for instance, methane at an overall pressure of 1 atmosphere absolute. This results in a lowering of the system pressure and in the escape of ammonia and some carbon dioxide, so that a mixture of methane, ammonia and carbon dioxide with an overall pressure of 1 atmosphere absolute is obtained. In order to remove traces of carbon dioxide contained in the gas mixture, part of the mixture is condensed, causing the carbon dioxide to be absorbed by the liquid ammonia.
U.S. Pat. No. 4,060,591 discloses a process for recovering ammonia from aqueous mixtures also containing CO.sub.2 and H.sub.2 O wherein the mixture is first deacidified by stripped out CO.sub.2 at an elevated pressure. The remaining liquid is then stripped of all NH.sub.3, CO.sub.2 and H.sub.2 O, and the resulting gas mixture is scrubbed or stripped at a reduced pressure, relative to the deacidifcation step, to yield a gas stream of pure ammonia.
A similar process is described in British Pat. No. 916,945 wherein an ammoniacal liquor is deacidified at an elevated pressure in a column wherein the ascending gas is washed of ammonia by cold water, thereby yielding gaseous carbon dioxide. The remaining liquid is thereafter expanded into a stripper wherein it is freed of all ammonia and carbon dioxide. The resulting gaseous mixture is washed or scrubbed with all or a portion of the ammoniacal liquor feed, before the latter is deacidified, yielding a substantially pure ammonia stream.
In British Pat. No. 1,129,939, a gas mixture consisting of ammonia and carbon dioxide, rich with respect to ammonia, is absorbed in water or an aqueous solution. Ammonia is distilled from the resulting aqueous solution at atmospheric pressure. The remainder of the solution is then subjected to fractional distillation at a pressure of between 5 and 20 atmospheres absolute with heating in order to remove the carbon dioxide.
These latter processes are based on the principle that changing the pressure of a system of ammonia, carbon dioxide and water makes it possible to separate out ammonia at the lower pressure and carbon dioxide at the higher pressure. In these "pressure differential" processes the system pressure in the carbon dioxide separation zone should be at least twice that in the ammonia separation zone. Preferably the ratio between the system pressures in the ammonia separation and the the carbon dioxide separation zones should be between about 1:5 and 1:20, if the separation is to proceed smoothly.
However, the pressure differential processes have the drawback that if the ammonia and carbon dioxide mixture is available at a pressure of more than 1 atmosphere, it first has to be expanded to 1 atmosphere. Gaseous ammonia is then released having a maximum pressure of 1 atmosphere, or even lower in the event a large amount of another gas is present. If this ammonia is to be subjected to further processing, such as in a urea synthesis process, it has to be raised to a higher pressure. The compression energy required for this is quite substantial. Furthermore, the carbon dioxide concentration in the ammonia has to be kept extremely low in order to avoid the formation and deposition of solid ammonium carbamate in the compressor and high pressure lines.
An alternate process not requiring this pressure differential is disclosed and claimed in Biermans U.S. Pat. No. 4,163,648, the entire disclosure of which is hereby incorporated by reference herein. The process therein described permits the recovery of ammonia and carbon dioxide separately from such mixtures, without the need for such a pressure differential, if the ammonia and carbon dioxide containing feed supplied to a carbon dioxide separation zone is diluted by the addition of water in an amount of between 0.2 and 6 times, by weight, the total weight of such ammonia and carbon dioxide containing feed. For simplicity this latter process will be referred to herein as a "dilution process."
According to one embodiment of the dilution process, ammonia substantially free of carbon dioxide and water is first separated from a mixture of ammonia, carbon dioxide and possibly water, rich with respect to ammonia, in an ammonia separation zone. From the residual liquid phase leaving the bottom of this ammonia separation zone, carbon dioxide is separated in the carbon dioxide separation zone, wherein the residual liquid phase from the ammonia separation zone fed to the carbon dioxide separation zone is diluted with between 0.2 and 6 times its weight of water.
According to another embodiment of the dilution process, carbon dioxide substantially free of ammonia and water is first separated in a carbon dioxide separation zone from a mixture of ammonia, carbon dioxide and possibly water, which mixture is lean with respect to ammonia. From the bottom of this carbon dioxide separation zone, the residual liquid phase is fed to a desorption zone wherein virtually all ammonia and carbon dioxide are desorbed and the resulting gas phase is introduced into the ammonia separation zone. Ammonia, substantially free of carbon dioxide and water, is recovered from this resulting gas phase in the ammonia separation zone, and the resulting liquid phase is supplied to the carbon dioxide separation zone. Diluting water is added to this carbon dioxide separation zone in an amount of between 0.2 and 6 times, by weight, the combined total weight of the initial mixture to be separated, plus the residual liquid phase from the ammonia separation zone fed into the carbon dioxide separation zone.
According to another embodiment of the dilution process, where the ammonia and carbon dioxide containing mixture to be separated also contains a substantial quantity of water, it may be advantageous to feed this mixture initially to the desorption zone wherein the ammonia and carbon dioxide are desorbed, and, together with some water vapor, are introduced into the ammonia separation zone. Ammonia, substantially free of carbon dioxide and water vapor, is obtained from the top of the ammonia separation zone, and the residual liquid phase, containing ammonia, carbon dioxide and water, is introduced into the carbon dioxide separation zone. Diluting water is also introduced into the carbon dioxide separation zone in an amount of between about 0.2 to 6 times, by weight, the total quantity of the residual liquid phase from the ammonia separation zone fed into the carbon dioxide separation zone. Carbon dioxide, substantially free of ammonia and water, is obtained from the top of the carbon dioxide separation zone, and the residual liquid phase from the bottom of the carbon dioxide separation zone is fed to the desorption zone.